Dehumidification Kiln

[Rob Will]

Has anyone constructed a dehumidification kiln?

Here's a general question:
Starting with green lumber, mostly cherry, should I air dry (in an unheated shed) the 5/4 lumber for a year or more before finishing off the drying process in some sort of kiln?

Thanks for your suggestions,

Rob

[Robert Mickley]

In my opinion, yes. My brother worked for an old fella sawning lumber for a while. They dried all their lumber this way. He had some of the nicest lumber around. He had furniture builders that would not buy anywhere but there. If he didn't have it available some of them just waited.

I've always been of the opinion the slower it dries the better it is.

[Bruce Wrenn]

There have been numerious articles on building a dehumidification kiln Last one I remember was in American Woodworker about three years back. A better choice might be a solar kiln, as you can dry wood from "wet" to 6-8 percent MC in 6-8 weeks. Only the fans need electricity. Recently, either Wood, or American Woodworker showed how to build a small version of a solar kiln. Main thing is to not rush drying in first stages, as this will cause problems.

[Keith Outten]

Rob,

I operated an Ebac dehumidification kiln for a dozen years. I built the box from the plans that Ebac provided when I purchased the unit.

In the beginning we experimented with air drying for various periods however my goal was to produce the best quality lumber and production was not a big concern. Air drying really dropped the time in the box but lowered the quality. In the end I purchased lumber from a local mill right off the saw then rushed it back to the Kiln, closed the door and started the drying cycle. The first day I only ran the fan, then started the unit then last the heater to start bringing up the temp. The temperature in the kiln would start upward as soon as I closed the door by a couple of degrees just using the fan to circulate the air in the box.

Wet lumber would complete the entire drying cycle without the ends of the boards splitting....ever. I also noted that wet lumber had no internal stresses at the end of the cycle...ever. The longer I air dried the more stress I found in the lumber at the end of the drying cycle, not much but enough that you could tell the difference. I always dried local species so my information is limited to oak, walnut, poplar, ash, hickory and maple for the most part.

Air drying will easily double or triple production by reducing time required in the kiln.

My 2 cents.

.

[Rob Will]

What about making a drying kiln from a 12' refrigerated truck body fitted with a modern dehumidification unit?

I would rather load it from the side with a forklift but if need be I'll hand load it or build a cart for end loading. Annual volume does not need to be all that much considering that this is for my own use only. The maximum length that I need to dry is 10'.

Any suggestions?
Rob

[Cody Colston]

I have the American Woodworker issue with the dehumidification plans. IIRC, that kiln does not use heat, only dehumidification to dry the lumber. It also goes through the "conditioning" process necessary after it is dried in order to relieve any internal stresses.

The WOOD solar kiln is supposedly almost foolproof, in that you load it up and basically forget it until dry. There's no measuring of moisture removed or adjusting of the dehumidification timer according to their article. It seemed to me to be a combination of passive solar heat and natural convection.

One day I plan on building some sort of kiln. I'm interested in hearing more on the subject.

[Dan Invan]

I built the AWW kiln (from the June '02 issue) in the fall, following the plans fairly closely other than making it a full 8' long. I live in the northwest, there is not enough sun here for a solar kiln other than in the summer.

Some things I can pass along are to make sure that you use decent plywood, the stuff I bought was crap and I wound up having a terrible time with the fit. Especially of the door. Also make sure you insulate the floor of the kiln, if you have it in an unheated garage as I do. Be generous with the caulking! I also built a simple extender for the DH control potentiometer which exits via a small hole, and has a knob mounted to it externally. This is slightly different than what the plans called for but resulted in a smaller hole.

I bought a decent (Electrophysics) moisture meter, and also a cheap humidity/temp gauge from Radio Shack, both to measure progress. The type with a wire probe so it can read outside temp, as well as inside temp and humidity. They are cheap and if you pop them open you can see the sensors used for the inside temp and humidity, it's easy to solder them out and resolder them at the end of long wires so now you have a gauge that'll read the kiln inside temp in two places as well as the humidity. It's quite handly to be able to read it all from the outside, no need to open the kiln or install some kind of goofy window. If you can measure temp, RH, and the wood's moisture content, you should in theory not require a wet bulb temp reading. That's my theory and I'm sticking to it (so far)!

With the control electronics, the plywood, paint, moisture meter, DH, fans, etc etc I am probably into this kiln for close to $1000, never mind all the time I spent building it.

One problem I quickly uncovered in the AWW article was the drying rate info they provided was for a limited number of species. Woodweb has some info, but I am still working on getting more useable info on drying schedules. I find the commonly available ones to be unfriendly.

Operationally I found that I was having a hard time getting the temperature up high enough at the start of the cycle. So I insulated the floor and increased my lightbulb heater wattage. I eventually found that running the DH throws way more watts into the kiln than just the lightbulbs do, so temp control is tricky. Once the RH drops and the DH is running constantly, the temp may spike up quite high. Also, the temp is not uniform throughout the kiln, so the temp that the controls are seeing is not necessarily the temp the wood is experiencing. Temp is my main operational problem.

The DH operates like a champ, it pulls the water out quite readily, I measure the water bucket every day (OK almost every day), as well as all the other readings, and record them. I seem to max out (or more correctly I suppose 'min out') at around 9% MC. Over the drying cycle I find the temp bounces around quite a bit but trends up, the amount of water pulled out drops off quite quickly so the DH must be turned up almost daily, the MC also bounces around a little bit, but generally trends down quite smoothly, and the RH does not move much at all. I also note that the inside temp also moves up and down somewhat with the outside temp, despite the insulation. When temps here dropped into the 20's a few weeks ago, the poor little kiln was really hurting to try and get much above 70.

I do a "conditioning" cycle once the wood is dry, I hose it down and run it for another week.

Usually it takes about 5-6 weeks total to do a load that is partially air dried. I have had decent results, no casehardening problems, modest amounts of cupping/warping/twisting. The biggest problem has been cracking and splitting, plus cupping on some really huge slabs.

Since I resaw all my own wood, either with an alaska mill or on my bandsaw, I wind up with a little of this and a little of that, so I have been doing mixed loads. Which all the pros say is a no no. But I just use the schedule for the 'slowest drying' wood in the load, and reduce it a bit more just to be safe . . .

Anyways, there are some of my experiences.

[Travis Porter]

Good information!

[Jim Becker]

For your air drying period...which is a good idea, BTW, as it will cut down on your energy usage in the kiln and generally provide nicer results...drying in an open shed is fine, but you don't want the wood in a place that has no air flow. You need the air blowing through the stack to wick off the moisture evenly. The only reason for a "cover" is to keep standing rain and snow off the wood.

[Russ Massery]

Rob,
The Guy I buy most my lumber from in Michigan who uses a Dehumidification Kiln. I found all the said above to be true. It's the most stress free wood I have ever work with. He bought two old semi trailers had them sprayed with insulating foam. Then installed the Dehumidification unit in them. I believe he said he air dries the wood to about 20% or so before putting it in the kiln. If you have any question about the process here Steve's website. I'm sure he would glad answer any of your questions. http://www.hickoryandoak.com/lumber.htm
He's great guy to do business with. I can't say enough about him.

Russ

[Rob Will]

Rob,
The Guy I buy most my lumber from in Michigan who uses a Dehumidification Kiln. I found all the said above to be true. It's the most stress free wood I have ever work with. He bought two old semi trailers had them sprayed with insulating foam. Then installed the Dehumidification unit in them. I believe he said he air dries the wood to about 20% or so before putting it in the kiln. If you have any question about the process here Steve's website. I'm sure he would glad answer any of your questions. http://www.hickoryandoak.com/lumber.htm
He's great guy to do business with. I can't say enough about him.

Russ

That is an interesting site. I wonder how he slices those monster logs with that circle mill?

Rob

[Ted Christiansen]

Dan,

Thank you for that great information. I have that AWW article and plan to build the kiln, also 8' long. Do you happen to still have the part number of the Radio Shack temp/humidity sensor?

I also mill my own lumber with a chainsaw mill (Husky 385XP, Logosol bars, chains and Woodworkers Mill). I have airdried both outdoors and in my basement. In the basement basically uses the house as a kiln, which has been very successful on walnut, ash, cherry, red oak and spruce.

Ted

[Russ Massery]

That is an interesting site. I wonder how he slices those monster logs with that circle mill?

Rob, I believe he Quarter's them first. That's why it's easy to get Quartersawn lumber from him.

Russ

[Peter Lyon]

Sorry for momentarily diverting from what is proving to be an interesting topic -- but holy cow the prices for hardwood at the aforementioned mill are simply unbelievable for this westcoaster.

[Russ Massery]

Oh yeah, imagine my excitment when I found out about him.